I think I can see why this was not used. While stainless steel is corrosion resistant, and springy, it isn't used for making contacts. Only two metals are normally used for electrical contacts: gold and tin. Other corrosion-resistant metals, like aluminum, usually work by forming a very thin and transparent - but insulating - oxide layer on their surfaces.
Iron, though, is the metal for making springs. And springs flex. So the plating on a gold-plated spring would normally end up flaking off.
This has given me an idea, though, but this idea is also unworkable. If springs are made of iron, while iron is not a good electrical contact material, iron has another useful property: it can be magnetized.
So have a magnetized spring, and when it buckles, it causes a reed switch to close, or actuates a Hall-effect detector!
However, while those types of magnetic switches have been used in keyboards, they all required the magnet to move vertically with the plunger. In a buckling-spring design, the magnetic element wouldn't move by as large a distance when the key is pressed, so it probably wouldn't be possible to make a keyswitch with reliable "closed" and "open" positions.